It is known to use a capacitance type transducer for the measurement of acceleration and other physical quantities such as pressure, fluid level, humidity, position and/or proximity of various items. In the case of at least accelerometers and pressure sensors a movable capacitor element is suspended by a resilient support near a fixed capacitor element and a force on the movable element caused by acceleration or pressure displaces the movable element against a restraining force thereby changing the capacitance. A measurement of the capacitance or the change of capacitance can then be used as a measure of the displacement and therefore of the acceleration or the pressure.
In such transducers two principle types are used depending on the restraining force. In open loop devices, the spring rate of the support provides a force which counters the input force and establishes an equilibrium position. In force balance devices, an electrical feedback signal is generated and applies a restraining force by magnetic or electrostatic attraction of the movable capacitor element. The advantage of the force balance devices is that the movable element can be held to a very small displacement, thus avoiding fatigue of the support and the consequent change of spring rate over the life of the transducer.
Typically, a capacitive accelerometer has a moveable proof mass constituting a capacitor electrode suspended between two other capacitor electrodes for movement along the axis in which acceleration is to be sensed. In the interest of compactness and integration with associated circuitry, such devices may be micromachined silicon variable capacitors. An example of such technology is shown in the article "Airbags Boom When IC Accelerometer Sees 50G", Electronic Design, Aug. 8, 1991, pp 45-56, which is incorporated herein by reference. The circuit described in that article is adaptable to both open loop and force balancing transducers. However it requires large capacitors not amenable to an integrated circuit and thus are "off-chip". The circuit further requires AC input excitation and is sensitive to frequency variations, and the balancing feedback has a high resistance coupling into the accelerometer capacitance, thus giving rise to temperature sensitivity problems.
My U.S. Pat. No. 4,467,655, entitled "Quad-Gate Demodulator Circuit for Capacitive Transducer" is an example of a circuit suited for an open loop transducer and it is not adaptable for use with force balancing. It uses quad-gate switching and it requires an AC input signal and a demodulating scheme as well as two large coupling capacitors which are external capacitors when the device is an integrated circuit.
It is desirable to provide a transducer circuit which is applicable to both open loop and force balancing transducers with only minor modification, requires no large capacitors, and is simple.